Chapter 5 - L: Live Loads
© 2007, T. Bartlett Quimby
Roof Live Load Reduction
Last Revised: 09/11/2008
The nature of roof live load is different from that of floor live load so the approach to reducing roof live load is different from that used for floor live load. The requirement is presented in ASCE 7-05 4.9. The commentary in ASCE 7-05 C4 about roof live load reductions explains the anticipated difference in use of roof areas vs. floor areas.
The reduction formula has two reduction terms: One for the tributary area supported by the structural element (R1) and the other for the slope of the roof surface (R2).
R1 is formulated in such a way that there is no reduction for tributary areas less than 200 sqft and is maximum when the tributary area reaches 600 sqft, varying linearly in between. For use in spreadsheets, the formula can be written as:
R1 = max[min[1.2-0.001AT, 1], 0.6]
R2 is formulated in such a way that there is no reduction for roof with slopes that have a slope ratio 4:12 or less. It is maximum for slope ratios of 12:12 or greater. The formula can be written as:
R2 = max[min[1.2-0.05F, 1], 0.6]
Where "F" is the rise in the slope ratio F:12.
This slope reduction method for roofs assumes a normal roof that is not expected to have any significant occupancy. If the roof has such an occupancy, then the area reduction for roof live loads may not be taken.